Semiconductors such as integrated circuits are formed on wafers which are then cut into dice or chips that individually may be mounted onto substrates. Usually the substrate is electrically and thermally conductive, and mounting provides both good electrical and good thermal conductivity between the die and the substrate. Good thermal conductivity permits the substrate to serve as a heat sink for the die. An effective heat sink is desirable, because as a rule of thumb, an increase of 10.degree. C. in the operating temperature of a semiconductor die reduces its life by one-half. Generally the attainment of good thermal conductivity is inherently accompanied by adequate electrical conductivity.
Of the two prevalent techniques for attaching a semiconductor die to an electrically and thermally conductive substrate, one employs a solder or eutectic alloy such as a gold-silicon alloy. While such alloys provide excellent electrical and thermal conductivity, they are exceedingly expensive. Also, their application temperatures may be so high that some dice might be damaged, and differences in thermal expansion sometimes cause a die to break. Furthermore, it is difficult to handle an individual die together with an alloy strip of the same size.
Because of the expense and difficulties in using eutectic alloys, it has become more common to employ a spreadable die attach adhesive consisting of a heat-curing epoxy resin composition filled with fine metal particles, usually silver and occasionally gold. Spreadable epoxy adhesives can be difficult to use, especially those which are marketed in two parts and must be thoroughly mixed shortly before use. Care must be taken to keep the metal particles in suspension, to spread out a uniform layer of the proper thickness, and not to allow voids in the adhesive layer. Automated attachment of dice to substrates is complicated by the waiting period during which the epoxy composition is cured.